1. Field of the Invention
This invention relates to a process for making a helmet, and more particularly to a process for making a helmet for a motorcyclists, cyclists, skaters, etc.
2. Description of the Related Art
U.S. Pat. No. 5,376,318 disclosed a process for making a helmet with a plurality of ventilation holes for cyclists. Such a process suffers from the following disadvantages:
1. Each of the molds must have a heater and suction device mounted thereto in order to pre-heat the upper and lower dies of the mold and to deform the plastic film. Therefore, a number of heater and suction devices are required in the process, resulting in a high manufacturing cost. In addition, the formation of the bottom through holes in the lower dies increases the manufacturing cost.
2. The printing ink cannot be coated uniformly on the plastic film. The plastic film can only be printed with one color and cannot be printed with regular patterns.
3. Since the liquefied polyurethane mixture is not shaken sufficiently when being filled into the lower die, well foaming of the polyurethane cannot be achieved. The density of the foamed polyurethane is not uniform and gas bubbles are formed in the surface of the foamed polyurethane. In addition, because there is no flash gate in the upper die, the air holes in the upper die is liable to be blocked by the foamed polyurethane.
4. Since the cavity of the lower die is covered with the plastic film which has no air holes and there is no hole-making steps after the polyurethane mixture is foamed, gas bubbles is liable to formed between the contact faces of the plastic film and the foamed polyurethane. Therefore, the density of the helmet thus obtained is not uniform.
5. The printing ink cannot be coated uniformly to the shape-molded plastic shell because of configuration of the cavity of the lower die. Therefore, the color formed on the external surface of the helmet is not uniform. In addition, because the shape-molded plastic shell has not air holes, gas bubbles are liable to form between the contact faces of the foamed polyurethane and the shell.